Chain Reorganization Mitigation on ETC

In August 2020, Ethereum Classic experienced multiple 51% attacks that resulted in deep chain reorganizations and double-spend attempts. The community responded with MESS (Modified Exponential Subjective Scoring), a novel defense mechanism.

## The 2020 Attacks

Three separate 51% attacks occurred in August 2020:

- **August 1**: 3,693 block reorganization (~12 hours of chain history rewritten)
- **August 6**: 4,000+ block reorganization
- **August 29**: 7,000+ block reorganization (the deepest)

The attacker used rented hashrate to build alternative chains in secret, then broadcast them to reorganize the canonical chain. This enabled double-spend attacks against exchanges.

## MESS: The Response

ECBP-1100, known as MESS (Modified Exponential Subjective Scoring), was implemented in October 2020. The mechanism works by penalizing chain reorganizations that go back many blocks:

### How MESS Worked
- Each node tracks the "gravity" of the chain it's following
- A competing chain that reorganizes N blocks back must demonstrate exponentially more work than the current chain tip
- Short reorgs (1-2 blocks, normal PoW behavior) are unaffected
- Deep reorgs (100+ blocks) become effectively impossible even with majority hashrate

### Key Properties
- **No consensus change**: MESS is a node-level policy, not a consensus rule
- **Gradual penalty**: The penalty increases with reorganization depth
- **Configurable**: Node operators could adjust or disable the penalty

## The Merge Changes Everything

When Ethereum transitioned to proof-of-stake in September 2022, GPU miners migrated to ETC in massive numbers. Network hashrate increased from ~25 TH/s to over 200 TH/s — an 8x increase.

This dramatically changed the economics of 51% attacks. The cost to rent sufficient hashrate to attack ETC became prohibitively expensive.

## Deprecation

With the post-Merge hashrate providing strong natural protection, MESS was deprecated. The community determined that the economic security from high hashrate made the additional subjective penalty unnecessary — and removing it simplified the client codebase.

## Lessons Learned

The 2020 attacks and MESS response demonstrated ETC's ability to:
1. Respond quickly to security incidents
2. Implement novel defense mechanisms
3. Remove temporary measures when they're no longer needed
4. Rely on fundamental PoW economics for long-term security